Heat shields are used for example in combustion chambers or flame tubes, which may be part of a kiln, a hot gas channel or a gas turbine and in which a hot medium is produced or guided. Gas turbine combustion chambers that are subject to high levels of thermal loading are thus for example lined with a heat shield to protect against excessive thermal stress. The heat shield typically comprises a number of heat shield elements disposed over a large area of a supporting structure to shield the walls of the combustion chamber from the hot waste gases from the combustion process. In order not to impede the thermal expansion of the heat shield elements on contact with the hot waste gases from the combustion process, said elements are fixed to the supporting structure leaving gaps between adjacent heat shield elements.
Such a heat shield on a supporting structure is for example disclosed in EP 0 558 540 B1. In this heat shield ceramic heat shield elements have a hot side to face the hot waste gases, a cold side opposite the hot side and four peripheral sides connecting the hot side to the cold side. Two peripheral sides facing away from each other have grooves, in which grip sections of securing elements can engage. The securing elements have a fixing section for fixing to the supporting structure and a securing head with the grip section. To fix the heat shield elements to the supporting structure, the fixing sections are fixed to the supporting structure and the grip sections of the securing heads are made to engage with the grooves in the heat shield elements.
The securing elements are made of metal and have spring characteristics. The spring characteristics allow the securing head to yield when the heat shield elements expand due to thermal effects, thereby preventing the formation of cracks in the heat shield elements or fracturing of the securing elements. Also the spring effect allows movement of the heat shield elements in relation to the supporting structure within certain limits.
So that the thermal expansion and/or the movement of the heat shield elements is not impeded by adjacent heat shield elements, in EP 0 558 540 B1 these are disposed leaving gaps between adjacent heat shield elements. However hot gas can penetrate through the gaps into the heat shield in the direction of the metal securing elements. As the metal securing elements are generally less able to tolerate thermal loading than the ceramic heat shield elements, the gaps are flushed with cooling air, to prevent penetration of the hot gas into the gaps. Flushing results in a mass air flow, which enters the combustion chamber through the gaps and seals the gaps against penetration of the hot gases. A channel for supplying a cooling fluid is assigned to every securing element for cooling purposes. The sealing of the gaps between the heat shield elements is not however regular, meaning that more cooling air is necessary for reliable sealing than would theoretically be required for sealing the gaps.
The mass air flow required to seal the gaps is not available for combustion purposes and has an adverse effect on the potential for NOx minimization. Also the geometry and arrangement of the securing elements make effective cooling of the securing heads exposed to the hot gas problematic.